was initiated to investigate the technical Greenhouse gas sequestration and economic feasibility of CO2 stor- age in a partially depleted oil reservoir (Government of Canada, 2000). The IEA in abandoned oil reservoirs: Weyburn project is exploiting EnCana Corporation’s $1.5 billion, 30-year com- The International Energy Agency mercial CO2 enhanced oil recovery operation, which is designed to recover an incremental 130 million barrels of oil Weyburn pilot project from the Weyburn field through the in- jection of gaseous CO2 under pressure. D.J. White, Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario K1A 0E9, Specifically, the IEA Weyburn Project Canada, [email protected] aims to comprehensively monitor and verify the progress of the CO flood G. Burrowes, EnCana Corporation, 421 7th Avenue SW, P.O. Box 2850, 2 and establish the likelihood of safely Calgary, Alberta T2P 2S5, Canada storing the CO2 in the reservoir for the T. Davis, Colorado School of Mines, Golden, Colorado 80401-1887, USA long term. Toward this end, a multidis- Z. Hajnal, Department of Geological Sciences, University of , ciplinary, integrated program has been , Saskatchewan S7N 0W0, Canada formulated to address critical issues cen- tral to safe and cost-effective, long-term K. Hirsche, Hampson-Russell Software, 715 5th Avenue SW, Calgary, storage of CO . In this article, we focus Alberta T2P 2X6, Canada 2 on the regional geoscience framework I. Hutcheon, Department of Geology and Geophysics, University of Calgary, and the monitoring and verification Calgary, Alberta, T2N 1N4, Canada components of the project. E. Majer, 90-MS1116, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA GEOLOGICAL SETTING AND HISTORY OF THE WEYBURN FIELD B. Rostron, Department of Earth and Atmospheric Sciences, University of Alberta, The Weyburn oil field is located Edmonton, Alberta T6G 2E3, Canada southeast of Weyburn, Saskatchewan, S. Whittaker, Saskatchewan Industry and Resources, 201 Dewdney Avenue E, within the north-central Williston Basin, Regina, Saskatchewan S4N 4G3, Canada which contains shallow marine sedi- ments of Cambrian to Tertiary age (Fig. 1). The Weyburn field, which covers ~180 km2, was discovered in 1954 and ABSTRACT INTRODUCTION hosted an estimated 1.4 billion barrels of Carbon dioxide sequestration in geo- Carbon dioxide is the primary anthro- oil. Primary production within the field logical reservoirs is being evaluated pogenic greenhouse gas in the modern- continued until 1964, when the initiation internationally as a viable means of day atmosphere and is a critical compo- of waterflood resulted in oil production long-term CO2 storage. The International nent in models of global climate change peaking at 46,000 barrels/day in 1965. Energy Agency Weyburn CO2 Monitoring (IPCC, 2001). It is estimated that ~6 gi- Waterflood has continued since then, and Storage Project is designed to inves- gatons of carbon enters the environment with horizontal infill drilling commenc- tigate the technical and economic fea- annually as a result of global energy-re- ing in 1991. Approximately 24% of the sibility of CO2 storage in a partially de- lated CO2 emission, with North America original oil in place had been recovered pleted oil reservoir in conjunction with being responsible for ~20%–25% of this by 2000 when CO2 injection began. enhanced oil recovery operations. Two total (International Energy Agency [IEA], Weyburn oil reserves reside within a key elements of the project are (1) the 2000). Recognition of the importance of thin zone (maximum thickness of 30 m) establishment of a regional geoscience CO2 emissions has stimulated research of fractured carbonates in the framework as a means for prediction toward mitigation of CO2 effects as man- beds (Fig. 2) of the Mississippian of the long-term fate of injected CO2, dated under the Kyoto Protocol of the Charles Formation, which were depos- and (2) development and application United Nations Framework Convention ited in a shallow carbonate shelf envi- of geophysical/geochemical monitoring on Climate Change. ronment. The reservoir comprises two and verification methods to track the CO2 sequestration in geological res- intervals, an upper Marly dolostone spread of CO2 within the reservoir. To ervoirs is being evaluated internation- (0–10 m thick) and lower Vuggy lime- 3 date, 1.90 billion m of CO2 have been ally as a viable means of long-term CO2 stone (0–20 m thick) that are sealed by injected into the reservoir, the effects of storage and climate change mitigation anhydritic dolostones and anhydrites of which are imaged by the various moni- (EAGE, 2000). In 2000, the IEA Weyburn the Midale Evaporite. The Midale Marly toring methods. CO2 Monitoring and Storage Project unit ranges from chalky dolomudstone

GSA Today; v. 14; no. 7, doi: 10.1130/1052-5173(2004)014<4:GGSIAO>2.0.CO;2

4 JULY 2004, GSA TODAY Figure 1. Three-dimensional map showing the location of the Weyburn study area in southeastern Saskatchewan. The dashed cube represents the area where the geoscience framework is being constructed. Lower inset shows the location of the International Energy Agency geoscience framework study area within the Williston Basin.

to calcitic biofragmental dolostone with (8%–20%) and higher permeability (10 1986) and is parallel to the horizontal intervening thin beds of biofragmental to >300 millidarcy). The higher permea- well direction. The vertical stress at the limestone. The fractured, Vuggy unit in- bility within the Vuggy unit resulted reservoir level due to the lithostatic load cludes a lower, peritidal “shoal” se- in preferential recovery of oil from this is ~34 MPa as estimated from a density quence with common secondary unit during the waterflood stage of log, and the minimum horizontal stress (vuggy) porosity, and an upper shallow production. is ~18–22 MPa in this region (McLellan marine “intershoal” sequence dominated The dominant fracture set within the et al., 1992). by fine-grained carbonate sands. The reservoir strikes NE-SW as determined Midale Marly has relatively high porosity from core and imaging logs (Bunge, THE CO2 FLOOD (16%–38%) and low permeability (1 to 2000). This orientation is sub-parallel to The CO2-based enhanced oil re- >50 millidarcy), whereas the Midale the regional trajectories of maximum covery (EOR) scheme was initiated in Vuggy has relatively lower porosity horizontal stress (Bell and Babcock,

Figure 2. A: Simplified regional hydrostratigraphy above the Frobisher sequence. The CO2 is conceptually shown migrating up-dip within the Midale beds, which subcrop at the Mississippian unconformity and the overlying Watrous aquitard. B: Representative schematic distribution of major lithologies within the Midale reservoir with approximate location indicated by the red rectangle in A. The trajectory of a horizontal (Hz) injection well is shown within the Midale Marly unit.

GSA TODAY, JULY 2004 5 September of 2000 in 19 patterns1 of the EnCana Weyburn unit at an initial injection rate of 2.69 million m3/day (or 5000 tonnes/day). The present rate of CO2 injection is 3.39 million 3 3 m /day of which 0.71 million m /day is CO2 recycled from oil production. The CO2 EOR is contributing over 5000 barrels/day to the total daily production of 20,560 barrels/day for the entire Weyburn unit. As of May 30, 2003, cumulative CO2 injected was 3 1.90 billion m . The CO2 flood will be expanded gradually over the next five years into a total of 75 patterns with ~10.8 billion 3 m (or ~20 million tonnes) of injected CO2 anticipated over the lifetime of the project. The source of CO is the Dakota 2 Figure 3. Subcrop and isopach of Gasification Company’s synthetic fuel plant located in Beulah, the reservoir and upper seal. . The CO2 is transported 320 km via pipeline to the Weyburn field. The reservoir is at ~1450 m depth in the EOR area and has a mean temperature of 63 °C. Estimated pore pressures of ~14 MPa existed when the field was originally discovered, and dur- ing waterflood they ranged from 8 to 19 MPa. Recently mea- sured pore pressures range from 12.5 MPa to 18 MPa with an average of ~15 MPa. These conditions exceed the critical point pressure and temperature (7.4 MPa and 31 °C) of CO2, and thus the injected CO2 initially exists as a supercritical fluid in the reservoir. It is anticipated that the injected CO2 will cause minor dissolution of carbonate minerals, as CO2 will dissolve to some extent in water present in the reservoir, forming bicarbonate until equilibrium is reached. This is ionic trapping of CO2. The presence of minor amounts of silicate minerals within the reser- voir may also enhance the capacity of the reservoir to sequester CO2 through mineral trapping as reactions between the silicates and CO2 may lead to the precipitation of carbonate minerals.

THE REGIONAL FRAMEWORK The motivation for developing the regional geological frame- work is to establish a means for predicting whether injected CO2 may migrate beyond the immediate injection site over the long term. To address this requirement, key elements of the re- gional framework that must be identified include potential fluid pathways, including faults, fracture zones and controlling depo- sitional features, such as zones of salt dissolution. CO2 traps and trapping mechanisms (e.g., seals and aquitards) within the Figure 4. A representative two-dimensional seismic profile from the stratigraphic column must be identified as well, and the trans- 1600 km of profiles that are being used with geophysical and core port properties of deep aquifers (e.g., porosity, permeability, logs to construct the regional geological framework. Horizons that are fracture distribution, formation-water flow rates and directions) picked regionally include the Precambrian unconformity, Deadwood, Winnipeg, Winnipegosis, Prairie Evaporite, Upper Backen, Upper need to be established. Watrous, Manville, Lower Colorado, and 2nd White spec. The The conceptual model of CO2 storage is shown in Figure 2. subvertical green lines represent interpreted lateral discontinuities in CO2 is injected into the Midale reservoir beds where the overly- the picked horizons that could be faults or fracture systems that may ing Midale evaporite forms the primary physical barrier to CO2 potentially act as fluid pathways. migration. However, the Midale reservoir beds are truncated to the northeast (as mapped in Fig. 3) where they subcrop regional three-dimensional geological framework (shown beneath redbeds (dolomitic to anhydritic siltstones and mud- conceptually in Fig. 1) is being constructed within an ~100 km stones) of the Watrous Formation. This forms a secondary seal. radius about the Weyburn field. The framework is based on Above the Watrous aquitard, there are several regional aquifers geological and geophysical compilations from the existing cat- sandwiched between thick shaley aquitards, which constitute alog of logged core from boreholes across the area, as well as the major flow units and flow barriers of the hydrostratigraphic reprocessing of 1600 km of seismic reflection profiles across framework. the area with stratigraphic control provided by the correla- To develop a detailed understanding of the geology and tion of seismic sections with geophysical logs. For example, hydrology from the Precambrian basement to the surface, a the subcrop and reservoir interval and upper seal isopach are

1Pattern refers to a group of injection and production wells that occupy an area of approximately 1 km2.

6 JULY 2004, GSA TODAY depicted in Figure 3. Similar maps are its spread will be influenced by het- porosity, permeability, fracture systems, being constructed for other key strati- erogeneous, anisotropic permeability, fluid distribution) prior to injection is graphic units. Figure 4 portrays an ex- and it will displace and mix with exist- important in planning the flood and an- ample of the regional seismic reflection ing reservoir fluids (oil, water, and gas) ticipating how it will proceed. Following data with horizons that are being picked resulting in pore pressure and fluid flood initiation, the goal is to track the as well as interpreted “faults.” The lat- composition changes within the reser- saturation and distribution of CO2 within ter may act as potential pathways for voir. Furthermore, the matrix rock of the the reservoir, assess the interaction of fluid migration, but the actual hydraulic reservoir will respond dynamically to the CO2 with the other reservoir fluids, characteristics of these zones (porosity, this process, potentially with associated determine pressure variations, and iden- permeability) are unknown. microseismicity signaling local deforma- tify off-trend flow so that the injection Regional fluid flow-direction, flow- tion (e.g., fracturing or pressurization of process can be adjusted accordingly. rates, and water chemistries of the 20 existing fractures). Typical magnitudes Finally, monitoring provides a means of major aquifers between the Precambrian of injection-related microseisms range verifying the volume of CO2 that resides basement and the land surface are being from –4 to 0 (Maxwell et al., 2003). The within the reservoir. Efficient and com- mapped in the Weyburn Project area. In preferred fluid pathways may in fact be plete access to the reservoir volume and addition, permeability and porosity data altered by the injection process as pres- avoidance of premature flow-through from core analyses and drill-stem tests sure changes within the reservoir may of CO2 to producing wells is important are being processed using geostatistical open or close fracture systems. whether enhanced oil recovery or CO2 tools to obtain hydraulic parameters for Monitoring of the CO2 flood at the storage is the ultimate goal. each aquifer. Weyburn field includes seismic and geo- chemical methods, which are intended Geochemical Monitoring RESERVOIR CHARACTERIZATION to document as much of this dynamic Chemical and/or isotopic composi- AND CO2 FLOOD MONITORING process as possible. Baseline static tions of aqueous fluids and gases are As CO2 is injected into the reservoir, characterization of the reservoir (e.g., being monitored and compared to

Figure 5. Contour map of the δ13C values from the Monitor 2 fluid sampling survey (July 2001). Black dots identify well locations where fluid and/or gas sampling was conducted within the nine-pattern Phase 1A flood area (dark outline). The larger, purple dots and horizontal well legs identify wells where a significant CO2 response was observed within four months following the sampling survey. For comparison, the red square identifies an area (shown in detail in the expanded inset panel, upper right) where the gamma parameter (or shear- wave splitting map) has been determined from the S-wave data for the Monitor 2 seismic survey (2002). The gamma parameter is a measure of the percent difference between the velocities of the fast and slow (split) shear waves and can be used to estimate fracture density and direction. In the inset, zones indicated by heavy black outlines identify negative anomalies where the gamma parameter map (not shown) values are <–10%. The heavy white dashed line is the salt dissolution edge.

GSA TODAY, JULY 2004 7 Figure 6. A: Horizontal crosswell attenuation tomogram for a sub-horizontal slice through the Marly unit (see Fig. 7 for orientation). The tomogram is determined for energy with a center frequency of 500 Hz. B: Permeability calculated using the attenuation values from A, based on the Biot relationship and using parameters (porosity and fluid viscosity) from the existing reservoir model. Units are in cm2 (1 darcy = 0.987 × 10–8 cm2).

Figure 7. P-wave amplitude difference maps for baseline minus 2001 survey (A) and baseline minus 2002 survey (B), determined from the three-dimensional P-wave surface seismic data. The amplitudes were determined as the arithmetic mean over a 5 ms window centered on the 6 3 reservoir horizon. The large circles represent the cumulative volume of CO2 (at reservoir conditions in units of 10 m ) that had been injected at the time of the monitor survey for each of the four dual-leg horizontal injectors. The large arrows indicate interpreted zones of off-trend CO2 spread. In A, the attenuation tomogram from Figure 6 is shown in an expanded panel emanating from its position on the amplitude map.

8 JULY 2004, GSA TODAY pre-injection baseline measurements to trace and predict the trends (cf. Li et al., 2001), suggesting the presence of off-trend movement of CO2 and other fluids within the reservoir. In zones of enhanced permeability. It should be noted, however, particular, carbon isotopes (δ13C ) are being used to track the that due to the acquisition geometry, the tomographic image spread of trace amounts of injected CO2 as a precursor of the cannot resolve permeable zones that are oriented parallel to advancing CO2 “front.” This is possible due to the distinct iso- the horizontal wells. 13 topic signature of CO2 (δ C value of –35 per mil) delivered from the synthetic fuels plant. Changes in fluid and gas com- Time-Lapse Multicomponent Three-Dimensional Surface positions over time indicate that interaction is taking place Seismic Monitoring Time-lapse seismic methods provide a powerful means of between reservoir fluids, injected CO2, and reservoir rocks. 13 monitoring the progress of the CO flood over time. The use For example, following the start of injection, the δ C (HCO3) 2 values of reservoir fluids have decreased dramatically from of multicomponent imaging techniques provides a means of original values ranging from –1 to –7 per mil to values of –4 isolating the various reservoir characteristics that are affected 13 by the flood (fluid distribution and saturation, pore pressure, to –11 per mil (Fig. 5). The decrease in δ C (HCO3) values is fracture permeability). For example, in a fractured porous most easily attributable to the dissolution of injected CO2. For geochemical monitoring surveys conducted at times greater medium that is isotropic (or for some classes of anisotropic than eight months following the onset of injection, a very media), the P-wave amplitude response is sensitive to both pore fluid saturation and pressure effects, whereas the S-wave good correlation has been observed between CO2 distribu- 13 amplitude response is generally less sensitive to the composi- tion (as estimated from δ C) and wells where CO2 has been detected in significant quantity within a period of four months tion of the pore fluid (e.g., Wang et al., 1998; Brown, 2002; following the survey (see Fig. 5). Cardona, 2002). Thus, P- and S-wave amplitudes provide a potential means of discriminating pore pressure vs. pore fluid Pre-Injection High-Resolution Seismic Crosswell Imaging saturation effects, except in regions of the reservoir where frac- Seismic crosswell data are intended to image variations ture-related anisotropy has lower symmetry (Cardona, 2002). in reservoir properties occurring on the scale of meters. Furthermore, it is reasonable to assume that the injected CO2 Combined with borehole sonic logs (centimeter scale), vertical will exist primarily as a supercritical fluid or in an oil-CO2 solu- seismic profile data, and surface seismic data (10s of meters tion, because of the relatively low solubility (~2 mol% at reser- scale), these data will provide proper scaling relationships for voir conditions) of CO2 in water and the relatively high reser- understanding reservoir properties and processes at the reser- voir pressures. This simplifies the interpretation of the seismic voir dimensions. response in terms of pore fluid composition. Rock property A horizontal crosswell survey was acquired in two paral- measurements indicate that the effect on seismic amplitude lel horizontal wells (Fig. 6) in August 2000, prior to the start of CO2 dissolved in oil is small until saturation levels reach of CO2 injection (Majer et al., 2001; Washbourne et al., 2001; 50% (Brown, 2002). Maximum CO2-induced P-wave velocity Li et al., 2001). A piezoelectric source was deployed in one decreases of 4%–6% are expected with associated reflection well and a 48-level hydrophone string in the opposite well. amplitude decreases of 15%–20% (Davis et al., 2003). The frequency band of the source was 200–2000 Hz, an order Birefringence (i.e., splitting) of shear waves provides another of magnitude higher than the surface seismic source. To our independent means of characterizing an anisotropic medium. knowledge, this was the first-ever deployment of a large-scale In the case of the reservoir, shear-wave anisotropy has been crosswell survey between horizontal wells. used to estimate fracture density, fracture direction, and loca- The horizontal crosswells were located within the Marly tion. It can also be used to monitor changes in the fractured unit (see Fig. 2B) of the reservoir. This layer has a much lower medium over time due to dynamic changes in the reservoir velocity (3.5 km/s) than the bordering layers (~5.5–6.0 km/s) processes. Davis et al. (2003) provide further details. and thus acts as a waveguide for seismic energy propagat- Time-lapse P-wave (2001 and 2002) amplitude difference ing across the layer. A tomographic imaging approach was maps for the first two monitor surveys relative to the pre- developed to use this guided or trapped wave energy. An at- injection baseline survey (2000) are shown in Figure 7. P-wave tenuation tomogram for 500 Hz energy is shown in Fig. 6A, amplitude anomalies are observed in the immediate vicinity of with a first attempt at integrating this result with the existing the horizontal injection wells. Generally, the areal extent of the reservoir model shown in Fig. 6B. The tomogram has been anomalies surrounding any of the four dual-leg horizontal in- converted from an attenuation image to permeability within jection wells is proportional to the cumulative amount of CO2 the depth slice by using porosity and fluid viscosity from the injected, whether the comparison is made for different injec- reservoir model, and using Biot relationships to calculate the tors in the same monitor survey or for the same injector in sub- permeability. The resulting permeability values clearly depend sequent monitor surveys. Preliminary volumetric calculations on the parameters from the reservoir model and the assumed and sensitivity modeling suggest that the P-wave amplitude attenuation mechanisms, but the observed trends should be anomaly maps are primarily mapping the CO2 saturation (pure robust. The permeability values obtained range from 50 to 150 phase and dissolved in oil) within the reservoir, with pressure millidarcy which is comparable to the range of permeabilities effects having a secondary influence. The limited extent of the measured within this unit (10 to 500 millidarcy). Of note, the amplitude anomalies within the S-wave map (Fig. 2 of Davis et spatial trends in the calculated permeability are at an angle al., 2003) also supports this conclusion, as the S-wave anoma- to the horizontal injection wells (oriented along-trend) and lies should be more sensitive to pressure effects over most of are also oblique to the local seismic impedance and porosity this area. Detailed reservoir modeling and flow simulation is

GSA TODAY, JULY 2004 9 being conducted to calibrate the seismic fractures), rock failure due to salt dis- Energy Research Institute, Saskatchewan results. solution in the underlying Prairie evapo- Industry and Resources, the European In addition to the main amplitude rite, or background seismicity associated Community, and ten industrial sponsors. anomalies, there are also smaller off- with the regional stress regime. Research is being conducted in North trend anomalies that suggest that chan- Short-term monitoring to date has America and Europe, including federal neling of the CO2 is occurring in some been unsuccessful in detecting signifi- and provincial government agencies, areas. One example of this is high- cant microseismicity at the Weyburn universities, and industry. This is publi- lighted in Figure 7 (see arrow), where field, in contrast to the experience in in- cation 2003161 of the Geological Survey an E-W spur is observed emanating jection projects elsewhere (e.g., Maxwell of Canada. from the main anomaly. A similar trend et al., 2003). Short-term monitoring was is observed on the spatially coincident conducted in 2001 for a period of sev- REFERENCES CITED attenuation (or calculated permeability) eral nights using the 48-channel hydro- Bell, J.S., and Babcock, E.A., 1986, The stress regime of the western Canadian Basin and implications for hydrocarbon image from Figure 6 (inset in Fig. 7) and phone array deployed within the reser- production: Bulletin of Canadian Petroleum Geology, v. 34, to a lesser extent on the S-wave am- voir for the horizontal crosswell survey p. 364–378. plitude difference map (Fig. 2 of Davis (see earlier section). Other attempts Brown, L.T., 2002, Integration of rock physics and reservoir simulation for the interpretation of time-lapse seismic data et al., 2003). These observations imply were made over a four-day cumulative at Weyburn Field, Saskatchewan [M.Sc. thesis]: Golden, the presence of enhanced permeability, period in 2000 and 2001 using the 12- Colorado, Reservoir Characterization Project, Colorado School of Mines, 208 p. which may be part of a larger pattern as level three-component downhole array Bunge, R.J., 2000, Midale reservoir fracture characteriza- described below. deployed for vertical seismic profiling tion using integrated well and seismic data, Weyburn Field, A curvilinear pattern of S-wave identi- within a 200 m interval directly above Saskatchewan [M.Sc. Thesis]: Golden, Colorado, Reservoir Characterization Project, Colorado School of Mines, 204 p. fied anisotropy occurs along the bottom the reservoir. To fully and finally assess Cardona, R., 2002, Topics on the seismic characterization fringe of the sub-area (red rectangle) microseismic levels at Weyburn, an of fractured reservoirs [Ph.D. Thesis]: Golden, Colorado, displayed in the inset of Figure 5, which eight-level array of three-component Reservoir Characterization Project, Colorado School of Mines, 218 p. correlates spatially with a similar pattern geophones has been cemented in place Davis, T.L., Terrell, M.J., Benson, R.D., Cardona, R., Kendall, 13 on the δ C map. This pattern generally ~200 m above the reservoir. Data acqui- R.R., and Winarsky, R., 2003, Multicomponent seismic char- follows the salt dissolution edge (white sition was initiated in August of 2003 acterization and monitoring of the CO2 flood at Weyburn Field, Saskatchewan: Leading Edge, v. 22, p. 696–697. dashed line in inset of Fig. 5) of the and is intended to continue for at least EAGE, 2000, European Association of Geoscientists and underlying Prairie evaporite, suggesting six months. Engineers, 62nd Conference, Glasgow, Programme and that a network of fractures may exist Catalogue, p. 16. CONCLUSIONS Government of Canada, 2000, IEA Weyburn CO2 within the reservoir in association with Monitoring Project gets funding, July 13, 2000, press re- salt dissolution. Alternatively, the ob- The results obtained to date within lease, www.nrcan.gc.ca (accessed December 2000). served anisotropic zone may be associ- the various elements of the Weyburn International Energy Agency, 2000, International Energy project are encouraging. The geosci- Agency Statement on the Energy Dimension of Climate ated with depositional facies-controlled Change, p. 1–28, www.iea.org (accessed December 2000). ence framework is at an advanced fractures as it approximately corre- Intergovernmental Panel on Climate Change (IPCC), 2001, sponds to the transition from intershoal stage of construction and will form Contribution of Working Group 1 to the third assessment the basis for numerical modeling of report of the IPCC, in Houghton, J.T., et al., eds., Climate to shoal depositional facies in the Vuggy change 2001: The scientific basis: Cambridge, Cambridge unit. In any case, both the seismic and the long-term fate of injected CO2. University Press, 892 p. geochemical results strongly suggest The monitoring methods (seismic and Li, G., Burrowes, G., Majer, E., and Davis, T., 2001, geochemical) demonstrate that the re- Weyburn field horizontal-to-horizontal crosswell seismic that the CO2 flood is advancing pref- profiling: Part 3—Interpretation [expanded abstract]: Society erentially along this zone of enhanced sponse of the reservoir to CO2 injection of Exploration Geophysicists 2001 Annual Meeting, San Antonio, 4 p. permeability. can be assessed and will be useful for Majer, E., Korneev, V., Daley, T., Li, G., Davis, T., the purposes of verifying the volume Washbourne, J., and Merry, H., 2001, Weyburn field Passive Monitoring of injected CO2. Ultimately, we antici- horizontal-to-horizontal crosswell seismic profiling: Part 1—Planning and data acquisition [expanded abstract]: Long-term monitoring of microseis- pate that the results from the Weyburn Society of Exploration Geophysicists 2001 Annual Meeting, micity is intended to help assess the dy- project will contribute to rigorous as- San Antonio, 4 p. namic response of the reservoir to CO2 sessment of the feasibility of using oil Maxwell, S.C., Urbancic, T.I., Prince, M., and Demerling, C., 2003, Passive imaging of seismic deformation associ- injection and may prove useful as an reservoirs for the sequestration of green- ated with steam injection in Western Canada [expanded alternate and/or complementary method house gases. abstract]: Society of Petroleum Engineers Annual Technical of flood monitoring. Microseismicity Conference and Exhibition, Denver, SPE 84572, 8 p. McLellan, P.J., Lawrence, K., and Cormier, K., 1992, A will be analyzed to constrain the loca- ACKNOWLEDGMENTS multiple-zone acid treatment of a horizontal well, Midale tion, magnitude, source mechanism, and This article was written on behalf of Saskatchewan: Journal of Canadian Petroleum Technology, v. 31, p. 71–82. likely geologic source and frequency of the Weyburn project, which is run by the Petroleum Technology Research Wang, Z., Cates, M.E., and Langan, R.T., 1998, Seismic occurrence of the characteristic seismic- monitoring of a CO2 flood in a carbonate reservoir: A rock ity and will be compared in detail with Centre of Regina, Saskatchewan, in physics study: Geophysics, v. 63, p. 1604–1617. Washbourne, J., Li, G., and Majer, E., 2001, Weyburn field the CO2 injection schedule and pro- collaboration with EnCana Resources horizontal-to-horizontal crosswell seismic profiling: Part 2— duction rate variability. Local seismic- (the operator of the Weyburn oil field). data processing [expanded abstract]: Society of Exploration ity might be anticipated, for example, Financial sponsorship of the project is Geophysicists 2001 Annual Meeting, San Antonio, 4 p. in association with CO2-induced rock provided by Natural Resources Canada, Manuscript received October 14, 2003; deformation (e.g., opening of existing the U.S. Department of Energy, Alberta accepted February 19, 2004. 

10 JULY 2004, GSA TODAY